A lidar (Light Detection and Ranging) in Patent Document 1 includes (i) a scanning means for transmitting a laser beam, and (ii) a photo detector for detecting reflected light of the transmitted laser beam and converting the reflected light to a reception signal. The lidar transmits the laser beam to a reference target such as a building, calculates an atmosphere attenuation based on the reflected light coming from the reference target which reflects the transmitted laser beam, and determines an amount of rain and a visibility distance based on the calculated atmospheric attenuation.
In the conventional lidar, it is necessary to transmit the laser beam to the reference target and detect the reflected light in order to determine the visibility distance. Therefore, the lidar cannot determine the visibility distance when there is no reference target such as a building around the lidar.                Patent Document 1: JP-2004-37474A        